The fabrication of a semiconductor device involves a plurality of discrete and complex processes. To perform these processes, a workpiece is typically disposed on a platen. The platen may be an electrostatic chuck, designed to retain the workpiece through the application of electrostatic forces produced by electrodes within the platen.
Platens are typically designed to be slightly smaller in diameter than the workpieces that they support. This insures that the platen is not exposed to the incoming ion beam. Contact with the ion beam could cause the generation of contaminants, or may do damage to the platen. Additionally, some platens have sloped or tapered side walls to minimize the possibility that the sidewall is exposed to the incoming ion beam.
In addition to retaining the workpiece in place, the platen may also serve to heat or cool the workpiece. Specifically, the platen is typically a larger mass of material, capable to drawing heat from the workpiece in some embodiments, or supplying heat to the workpiece in other embodiments. In certain embodiments, the platen has conduits on its upper surface which supply a back side gas to the space between the upper surface of the platen and the back surface of the workpiece.
Because the platen is somewhat smaller than the workpiece, the outer edge of the workpiece may not be heated or cooled as effectively by the platen. In fact, in some embodiments, the temperature near the outer edge may be 4-10% less than the rest of the workpiece. Further, the outer edge of the platen radiates more heat to the environment than the rest of the platen, which serves to lower the temperature of the platen at its outer edge. Thus, in embodiments where the platen supplies heat to the workpiece, the outer edge of the workpiece may be cooler than the rest of the workpiece. Conversely, in embodiments where the platen is removing heat from the workpiece, the outer edge of the workpiece may be hotter than the rest of the workpiece.
This difference in temperature may impact the yield of the workpiece. Additionally, these temperature gradients along the outer edge of the platen may exert thermal stress on the platen, which may lead to platen failure. Therefore, it would be beneficial if there were an apparatus to achieve better temperature uniformity across a workpiece, especially in embodiments where the workpiece is heated by the platen.